Quantification of intraskeletal histovariability inAlligator mississippiensisand implications for vertebrate osteohistology

Abstract

Bone microanalyses of extant vertebrates provide a necessary framework from which to form hypotheses regarding the growth and skeletochronology of extinct taxa. Here, we describe the bone microstructure and quantify the histovariability of appendicular elements and osteoderms from three juvenile American alligators (Alligator mississippiensis) to assess growth mark and tissue organization within and amongst individuals, with the intention of validating paleohistological interpretations. Results confirm previous observations that lamellar and parallel fibered tissue organization are typical of crocodylians, and also that crocodylians are capable of forming woven tissue for brief periods. Tissue organization and growth mark count varies across individual skeletal elements and reveal that the femur, tibia, and humerus had the highest annual apposition rates in each individual. Cyclical growth mark count also varies intraskeletally, but data suggest these inconsistencies are due to differing medullary cavity expansion rates. There was no appreciable difference in either diaphyseal circumference or cyclical growth mark circumferences between left and right element pairs from an individual if diaphyses were sampled from roughly the same location. The considerable intraskeletal data obtained here provide validation for long-held paleohistology assumptions, but because medullary expansion, cyclical growth mark formation, and variable intraskeletal growth rates are skeletal features found in tetrapod taxa living or extinct, the validations presented herein should be considered during any tetrapod bone microanalysis.

Maiasaura, a model organism for extinct vertebrate population biology: a large sample statistical assessment of growth dynamics and survivorship

“…The seemingly obvious comparison of dinosaur histology to that of their extant bird descendants does not apply here, because (with few exceptions), extant birds achieve adult size in less than a year and therefore do not deposit zonal bone. The 12 alligators used in this comparison were captive raised and therefore possessed growth rates much higher than their wild counterparts, and the nine oldest specimens had in fact attained skeletal maturity before their 26th year of life (Woodward et al 2011). Unfortunately, Alligator tibia data were not available to compare directly to Maiasaura tibia data.…”

“…3), with the simple purpose of testing the hypothesis that the growth histories of these two archosaur taxon will exhibit marked differences when directly compared in a normalized fashion. Alligators are a very well studied group histologically, and are often utilized to understand zonal bone growth in dinosaurs (e.g., Erickson 1996;Farlow and Britton 2000;Farlow et al 2005;Schweitzer et al 2007;Tumarkin-Deratzian 2007;TumarkinDeratzian et al 2007;Klein et al 2009;Farmer and Sanders 2010;Woodward et al 2011Woodward et al , 2014. The seemingly obvious comparison of dinosaur histology to that of their extant bird descendants does not apply here, because (with few exceptions), extant birds achieve adult size in less than a year and therefore do not deposit zonal bone.…”

“…Because the growth marks within an EFS are so closely spaced, accurately measuring the annual cortical growth is extremely difficult, so the cortical area within the entire EFS was divided by the number of growth marks, yielding equal annual growth for each year represented by the EFS. Although no alligator tibiae were available for comparison, results from Woodward et al (2014) demonstrate that Alligator mississippiensis femur cortical thickness increases are comparable to tibia cortical thickness increases, and that the femur and tibia are amongst the fastest growing alligator limb elements. In turn, Farlow et al (2005) show that Alligator femoral dimensions including midshaft circumference can be used to predict total length.…”

“…This normalization results in quantifiable measurements from Maiasaura individuals plotted on a dimensionless scale from 0% to 100% maximum size, with no need for potentially erroneous body size calculations. The same method was applied to a sample of twelve Alligator mississippiensis femora of known ages: ten captive male Alligator mississippiensis femora, representing nine skeletally mature 26 and 27 year old individuals with EFS (Woodward et al 2011) and one immature individual at least eight years of age; and two immature 5 year olds, one of which does preserve the first LAG, yielding size at age 1 year (Woodward et al 2014). A perinate Alligator femur was also measured to provide femur size at hatching.…”

“…The tibia was chosen for analysis because it lacks such a process and is therefore less subject to extensive remodeling. Although there is variation in the apposition rates of different bones within a skeleton (Horner et al 1999;Cullen et al 2014;Woodward et al 2014), this study samples only tibiae in order to directly compare variability present in a single element across multiple individuals. Along with the femur, the tibia tends to provide the most consistent skeletochronological results (Horner et al 1999), allowing reasonable estimates of maximum growth rates.…”